MODFQMMGR-746 Simplify migration code slightly

Author: mweaver-ebsco

src/main/java/org/folio/fqm/migration/strategies/AbstractSimpleMigrationStrategy.java

@@ -166,7 +166,7 @@ protected static String getNewFieldName(Map<String, String> fieldChanges, String
     } else if (fieldChanges.containsKey("*")) {
       return fieldChanges.get("*").formatted(oldFieldName);
     } else {
-      return fieldChanges.getOrDefault(oldFieldName, oldFieldName);
+      return oldFieldName;
     }
   }
 }